This invention relates to an apparatus and method for enhancing the release of conductive balls, eg solder balls, from a ball pick head in a ball transfer and placement apparatus. More generally the invention could also be applied to the release of any form of small object from a pick head or the like.
It is conventional to transfer and place on a ball grid array (BGA) substrate a plurality of solder balls simultaneously. This is done using a ball pick head which may comprise an array of openings matching the desired pattern of solder balls to be placed on corresponding solder pads of the BGA substrate. The openings are connected to a source of suction to hold the solder balls in place in the openings during the transfer process. The balls may then be released in a desired pattern on solder pads on the BGA. An example of such a conventional technique is described in the Assignee""s co-pending application Ser. No. 09/352,368. As shown in FIG. 1, a plurality of solder balls 1 are supplied to a ball template 4 in which they are arranged in a desired pattern before being lifted from the template 4 by a ball pick head 3 and moved along path A to a BGA substrate 7 where they are to be released. During the pick and transfer stages, the solder balls 1 are held to the pick head by suction, when the balls are to be released onto the substrate 7, this suction is removed to allow the balls 1 to be released from the pick head.
More generally, FIG. 2 illustrates two problems with conventional techniques. The left side of FIG. 2 illustrates the problem of excess balls adhering to the pick head when a pick head is lowered into a supply of solder balls that have not been pre-arranged in a desired pattern in the manner of FIG. 1. The second problem is illustrated in the right of FIG. 2. When the balls are to be released, they may not always be completely released as will be explained in the following.
Conventional techniques encounter difficulties with very small BGA packages such as chip scale/size packages (CSP) which are becoming increasingly popular with their use in small handheld electronic devices such as handheld computers, PDAs and mobile telephones. A CSP is a form of BGA with a substantially reduced form factor, and so the size of the solder balls is correspondingly reduced. Solder balls of a diameter of around 0.2-0.3 mm are conventional in CSP technology.
Unfortunately the size of such very small solder balls starts to introduce difficulties into the successful release of the solder balls from the ball pick head for a number of reasons. These include: (1) inevitable contamination by very small foreign objects on the surface of the balls or the lower surface of the pick head introducing an adhesive force such as surface tension between the balls and the lower surface of the pick head; and (2) electrostatic charges between the balls and between the balls and the pick head. While such phenomena may also be present in larger size balls and BGAs, they present more serious problems in CSP devices because the weight of the balls is much smaller and thus the gravitational force acting to release the balls from the pick head is correspondingly smaller. In this regard it will be appreciated that the weight of a ball decreases proportionately faster than the diameter of the balls. Because of these issues, reliable release of the solder balls from the pick head in the case of a very small BGA such as a CSP is difficult to achieve.
A number of attempts have been made in the prior art to overcome this problem and to enhance the release of very small solder balls from a ball pick head. A number of examples of such prior art and the approaches that have been taken will now be described.
One approach is described in U.S. Pat. No. 5,467,913 and is to apply a positive air/gas pressure to the inside of the ball pick head such as to help push or xe2x80x9cblowxe2x80x9d the balls away from the pick head on release. However, in practice only a small number of balls will stick to the head and the remainder will be released normally. This means that when the positive air pressure is applied it is wasted by being discharged through the openings from which the majority of the balls have already been released and it will have little or no effect on the small number of balls that are still adhered to the pick head.
Another approach is to move the pick head with the balls attached so that the balls contact the solder pads before they are released. Examples of this concept can be seen in U.S. Pat. No. 5,768,775, U.S. Pat. No. 6,099,681 and U.S. Pat. No. 6,158,649. The idea is that there will then be adhesive forces between the solder pads and the balls that will be greater than any forces tending to hold the balls to the pick head, and thus that these adhesive forces will assist in release of the balls from the pick head. However, in many cases it is difficult, if not impossible, to arrange for the solder balls on the pick head to be exactly co-planar and so when the pick head is moved to bring the balls into contact with the solder pads, some balls will contact the pads first and all forces will be concentrated through these balls with large stresses being exerted on them. This has the effect of increasing the adhesion of these balls to the pick head making it even harder to release them.
U.S. Pat. No. 6,170,737 describes an apparatus in which an array of pins is added within the pick head that act to push through the openings to release the solder balls from the pick head. This method works well for normally sized BGA designs, but with small-scale CSP devices it implies very difficult manufacturing processes and high cost.
Finally, further approaches to the problem include providing mechanical vibration either to the whole pick head or to the suction plate. U.S. Pat. No. 5,657,528 is an example of the latter. The suction template (in which is provided the suction openings holding the solder balls) is attached to the remainder of the pick head through vibration isolating members and a vibrating member is provided on the suction template to provide vibrations while the balls are being released. The drawback with this, however, is that the vibration pattern will depend on the pattern of suction holes which will vary between different semiconductor devices. This results in different vibration patterns with different BGA designs, and which different patterns may have quite different efficiencies at assisting release of the balls from the pick head. There will also inevitably be nodal points where there is minimum or even zero amplitude of vibration, and if any of these points exist at or near an opening with a solder ball, there may be insufficient vibration to secure the release of the ball.
A related approach is to apply a vibration to the entire pick head and not just to the suction template. However, this requires a relatively large vibration amplitude to be applied to the pick head which not only requires large vibration actuators but may also damage the pick head. Furthermore the vibration needs to be applied for a significant duration (a few seconds) which makes this method not very efficient.
According to the present invention there is provided a method for ensuring release of objects from a pick head in a transfer and placement apparatus, comprising causing said pick head to vibrate by applying a vibration signal to said pick head over a range of frequencies.
Preferably the vibration is applied by generating a random signal over said range of frequencies to a vibration means secured to said pick head, and the range of frequencies may be chosen to bound the principal resonant frequencies of said pick head. In this embodiment the resonant frequencies may be already known, or may be calculated by computer simulation. Frequencies above and below said range may be excluded by means of low and high pass filters.
According to another broad aspect of the present invention there is provided a method for ensuring release of solder balls from a ball pick head in a ball transfer and placement apparatus, comprising causing said pick head to vibrate by applying a vibration signal to said pick head over a range of frequencies.
According to a still further broad aspect of the present invention there is provided a method for ensuring release of objects from a pick head in a transfer and placement apparatus, comprising the steps of:
(a) analysing said pick head to identify the resonant frequencies of said pick head,
(b) selecting first and second frequencies defining a range of frequencies that include said resonant frequencies, and
(c) applying vibrations to said pick head over said range of frequencies.
Viewed from a further aspect the invention also provides apparatus for ensuring release of objects from a pick head in a transfer and placement apparatus, comprising: signal generating means for generating a random signal between first and second frequencies, vibration generating means for causing vibration of said pick head, said vibration generating means receiving said random signal as an input signal.